The invention relates generally to electromechanical switching devices such as relays or contactors. More particularly the invention relates to the armature or stator that is a part of the actuating mechanism.
Among the various mechanisms used to mechanically actuate electromechanical switching devices such as relays or contactors a commonly used form is the clapper mechanism. The clapper mechanism is named as it functions in a manner similar to that of clapping hands. One hand is movable and is called the armature. The armature is drawn by magnetic force to the second hand which is stationary and is referred to as the stator or core. An electromagnetic field is induced into the stator through the use of a coil that can be excited by either direct current (DC) or alternating current (AC). Application of a voltage to the coil will result in an electromagnetic field being induced in the stator which will attract the armature as the armature is comprised of a ferromagnetic material. As the armature is attracted to the stator it moves to the closed state for the device and actuates a mechanism which opens and closes electrical contacts in the electromechanical switching device. Removal of the voltage to the coil results in the loss of the electromagnetic field of the stator and the armature will move away from the stator under the influence of a return mechanism, usually comprised of a spring or other tension providing device, until it comes to rest in what is known as the open state. It is important to note that for the purposes of this disclosure the words “open” and “closed” refer to the state of the actuating mechanism for the device. Open being when the coil is de-energized and closed being when the coil is energized. Another usage for the terms “open” and “closed” is in relation to the electrical contacts that are operated by the clapper mechanism where the electrical contacts being controlled are commonly referred to as either Normally Open (NO) or Normally Closed (NC). For the purposes of this disclosure “open” and “closed” will refer to the state of the clapper mechanism, not the electrical contacts that may be controlled by the device.
Clapper mechanisms are designed with planar armature plates and planar stator cores that move about a fixed fulcrum point on the bottom of the armature plate. Upon energizing the coil, an electromagnetic field is created in the stator, and the armature is attracted to the stator and moves toward it until it comes to rest upon contacting the face of the stator. The armature is held in this position by electromagnetic force until such time when the coil is de-energized at which point the electromagnetic field collapses and the armature returns to the open state under the influence of the return mechanism.
In the art, the voltage at which the coil is energized is referred to as the “pull-in” voltage and the voltage at which the coil is de-energized is referred to as the “drop-out” voltage. Recall that the coil voltage induces an electromagnetic field in the coil and in turn the stator, thus below the pull-in voltage the electromagnetic field is insufficient to overcome the mass, friction, and return mechanism of the armature and move it into the closed position. At or above the pull-in voltage there will be sufficient electromagnetic field to overcome these elements and the clapper armature will be moved to the closed state. Conversely, in order to return the clapper mechanism to the open state the electromagnetic field must decrease to a point at which it can be overcome by the return mechanism and thus move the armature away from the stator pole face to the open position.
In the open position the planar armature is positioned with an inclination of a few degrees in relation to the flat pole face of the stator or core. This relationship describes a triangular shaped volume of air and defines the amount of travel required to close the clapper mechanism. Due to the size of the volume of air in the case where both the armature and stator have a planar face, the pull-in voltage must be high enough to generate an electromagnetic field sufficient to initiate the closing of the mechanism. The magnetic field starts out relatively weak though sufficient to initiate movement so the initial closing force is relatively low. However, as the armature moves toward the flat pole face of the stator the magnetic field rapidly increases and in turn the closing force until the armature contacts the pole face of the stator in the closed position. A problem with typical planar faced armature and stator embodiments is that this rapid increase of closing force overshoots the level required to close the clapper mechanism resulting in undesired wear and a decrease in the mechanical life of the device.
When the clapper mechanism is closed the magnetic field is at its strongest. Unfortunately the strength of the magnetic field in the closed state requires the drop-out voltage of the coil to fall to a very low level in order to allow the return mechanism to overcome the electromagnetic field and move the armature to the open state. The longer it takes for the coil to become de-energized the longer an electrical circuit that is being controlled by the contacts associated with the electromechanical switching device remain energized consequently presenting a potentially hazardous state to people or devices in addition to decreasing the service life of the device due to longer arcing times until the clapper mechanism moves to the open state and in turn de-energizes any circuits associated with the electromechanical switching device.
Thus there remains a need to increase the drop-out voltage within the tolerance band given by the relevant product standards in order to increase the speed at which a controlled circuit is de-energized improving safety while simultaneously decreasing the pull-in voltage resulting in a longer service life for these devices.